The present invention generally relates to solid state power controllers, and more particularly to secondary overload and alternate leakage current protection for solid state power controllers.
In electrical power distribution systems, such as those used to supply power to electrical loads on commercial aircraft, solid state power controller (SSPC) technology is gaining acceptance as an alternative to the combination of conventional electro-mechanical relays and circuit breakers. The advantages of SSPCs include their light weight, lower maintenance, better reliability, and small size. However, the “fail-closed” nature of the solid state device can be a safety concern. This concern has become a critical element for the certification of the SSPC technology for commercial aircraft and thus, a secondary means of protection is usually required.
Besides the “fail-closed” issue, leakage current is another potential disadvantage with SSPCs. When an SSPC channel is in the “open” state, maintenance personnel may come into contact with the open end of the power channel and get startled due to possible excessive leakage current. This potential safety risk becomes apparent during maintenance activities such as replacing an aircraft load that is connected to the SSPC channel with the excessive leakage current. The leakage current problem can be addressed by introducing an SSPC output clamping circuitry, which diverts the leakage current directly to the aircraft ground.
Safety and reliability requirements often necessitate the use of a secondary protection mechanism (e.g. a fuse) in series with the SSPC. Ideally, such a protection mechanism would provide the same level of wire protection, in terms of the energy rating, when the SSPC fails (closed). At the same time, this secondary protection mechanism should not interact with the SSPC's primary protection mechanism. That is, the secondary protection mechanism should interrupt power when required to avoid damage to downstream wiring, but should not interrupt power before the SSPC interrupts power during an over current/overload event. Because of this requirement, the selection of such a secondary protection mechanism and its coordination with the SSPC main protection mechanism (trip engine) can add significant complexities to the application of SSPC technology to commercial aircraft.
In particular, the secondary protection, such as a fusing means, must be carefully selected or designed to provide adequate protection to feeder wires yet also avoid the situation where the fusing means blows open before the SSPC trip engine can respond to circuit over current. There must be sufficient margin between the SSPC trip characteristic, and the downstream feeder wire's smoke limit to fit a fusing means characteristic for the secondary protection. In present applications, the margin has been realized by trade offs between fusing characteristics and minimum feeder wire sizes, often resulting in larger feeder wires and thus a heavier system distribution system. Another consideration is the “thermal memory” effect of the secondary protection, which needs to be accounted for by the SSPC trip engine. This adds even more complexity to the design of the SSPC trip engine, the selection of the secondary protection, and the selection of the feeder wire size. One consequence of the thermal memory effect of the secondary protection may be the use of an increased feeder wire size, which undesirably adds to the weight, volume and cost of the power distribution system.
As can be seen, there is a need for a way to provide a secondary means of overload protection for electrical power distribution systems containing SSPCs, with a closed-state failure mode. There is also a need for a circuit that can provide leakage current protection in electrical power distribution systems having SSPCs. There is a further need for a circuit that can facilitate the design of an SSPC trip engine by simplifying the selection of the secondary protection mechanism and the feeder wire size.